Prior to the present invention, various tube type fluid passes have been employed for heat exchangers such as an evaporator for an automotive air conditioning system. The core of such evaporators may employ a plurality of flattened plate passes operatively interconnected and each having discrete plate members sealed at their peripheral edges and at a mid rib to form a "U-shaped" refrigerant flow passage between inlet and outlet openings. These plates are also configured with bumps formed in patterns in the flow passage to vary the flow path of the refrigerant to improve heat transfer performance and to increase burst strength. While such constructions operate effectively, it is desirable to decrease the pressure drop across the heat exchangers and improve their heat transfer efficiencies.